Masking of exercise-induced ST-segment depression by rate-dependent left-axis deviation.

An unexpected high incidence of false negative ST-segment responses to exercise was previously reported for patients with coronary artery disease and abnormal left-axis deviation on their resting electrocardiograms. In the case presented, an exercise-induced, "ischemia" ST-segment depression was masked with the onset of rate-dependent left-axis deviation. The state of the base-line electrocardiogram is an important but often overlooked factor in assessing the incidence of false negative and false positive ST-segment responses to exercise.